An electrode using a hydrogen storage alloy capable of reversibly absorbing and desorbing hydrogen has a greater theoretical capacity density as compared with a cadmium electrode, and does not deform or cause the formation of dendrites in contrast with a zinc electrode. Therefore, it has a long cycle life and is pollution-free. Moreover, because of the high energy density, such an electrode has been frequently used as the negative electrode for a nickel-metal hydride storage battery and applied to, for example, the power source for small portable appliances, and is expected to undergo further development in the future.
Recently, the nickel-metal hydride storage battery has been attracting attentions as the power source for electric automobiles and the like, and there has been a strong demand for the improvement of the output characteristics and storage characteristics thereof.
In general, an MmNi5 alloy, where Mm is a mixture of rare earth elements, having a CaCu5 type crystal structure in which Ni is partly replaced by Co, Mn, Al, Cu or the like is widely used as the negative electrode material for the nickel-metal hydride storage battery. Additionally, researches and developments of a novel hydrogen storage alloy have been actively progressed in order to realize further miniaturization and weight saving for batteries.
When a produced hydrogen storage alloy is used as an electrode alloy powder as it is, the electrode performance is generally insufficient at the initial charge/discharge cycles. For this reason, it is preferable to conduct pre-treatments on the hydrogen storage alloy to improve the activity thereof, and various methods have been proposed for such treatments.
Heretofore, the following methods have been proposed as methods for improving the activity of a hydrogen storage alloy: a method in which components for improving the electrochemical activity of the hydrogen storage alloy, such as Ni, is physically applied onto the surface of the hydrogen storage alloy; and a method in which the surface of the hydrogen storage alloy is plated with Ni by means of, for example, a non-electrolytic plating. However, from the practical standpoint, a more promising method is the one in which the surface of the hydrogen storage alloy is chemically etched to improve the activity thereof since the method requires a relatively low cost. More specifically, the following methods have been disclosed:
(1) a method in which the hydrogen storage alloy is immersed in an aqueous acid solution such as hydrochloric acid (e.g., Japanese Unexamined Patent Publication No. hei 7-73878);
(2) a method in which the hydrogen storage alloy is immersed in an aqueous alkaline solution (e.g., Japanese Unexamined Patent Publication No. sho 61-285658); and
(3) a method in which the hydrogen storage alloy is immersed in an aqueous alkaline solution, followed by immersing in an aqueous acid solution (e.g., Japanese Unexamined Patent Publication Nos. hei 9-7591 and hei 9-171821).
The acid treatment in method (1) is not very effective in improving the activity of the hydrogen storage alloy, although it removes an oxide or hydroxide on the surface of the hydrogen storage alloy to improve the initial activity to some level. The reason is presumably because all the constituting elements of the hydrogen storage alloy are dissolved into the aqueous acid solution, so that an Ni-rich layer, which helps to improve the activity of the hydrogen storage alloy, is difficult to be formed on the surface of the hydrogen storage alloy. Additionally, the hydrogen storage alloy is exposed to an electrolyte of high alkali concentration in the alkaline storage battery. However, the dissolution behaviors of the constituting elements of the hydrogen storage alloy in an aqueous acid solution are different from those in an aqueous alkaline solution. The hydrogen storage alloy treated in the aqueous acid solution has a greater amount of the constituting elements to be dissolved into the electrolyte inside the battery than the one treated with the aqueous alkaline solution, and has accordingly shorter cycle life.
By the alkali treatment in method (2), only those unstable in the aqueous alkaline solution out of the constituting elements of the hydrogen storage alloy are dissolved into the aqueous solution, while the Ni-rich layer is formed on the surface of the hydrogen storage alloy. However, the dissolved elements are converted into an oxide or hydroxide to be deposited on the entire surface of the hydrogen storage alloy. Consequently, the conductivity of the hydrogen storage alloy is decreased, and the output characteristics of the resultant battery become insufficient.
By the combination of the alkali treatment and the acid treatment in method (3), the oxide or hydroxide deposited through the treatment with the aqueous alkaline solution are removed from the hydrogen storage alloy by the subsequent treatment with the aqueous acid solution, thereby improving the initial activity of the hydrogen storage alloy. However, this method requires a number of steps, such as an alkali treatment, washing with water, an acid treatment and another washing with water, presenting a problem of an increase in cost. Moreover, it presents another problem that the cycle life of the resultant battery becomes short, although not so short as in the case of the acid treatment in method (1), because the treatment with the aqueous acid solution is conducted at the end of the process.
The present invention relates to an electrode alloy powder which comprises a hydrogen storage alloy and is used for nickel-metal hydride storage batteries and the like, and to a method of producing the same.
The present invention is to effectively provide a highly conductive electrode alloy powder which is less prone to corrosion in an alkaline electrolyte and exhibits a superior electrode activity even at the initial charge/discharge cycles. By using the electrode alloy powder of the present invention, an alkaline storage battery of excellent self-discharge and high-rate discharge properties and a long cycle life can be obtained.
The present invention relates to a method of producing an electrode alloy powder, comprising: a first step of immersing a starting powder comprising a hydrogen storage alloy containing 20 to 70 wt % of Ni in an aqueous solution containing 30 to 80 wt % of sodium hydroxide at a temperature of 90xc2x0 C. or higher and; a second step of washing with water the powder which has been subjected to the first step.
The present invention also relates to the method, further comprising a third step of mixing the powder with an oxidizing agent in water after the second step.
The starting powder preferably has a CaCu5 type crystal structure and comprises an alloy containing a mixture of rare earth element, Ni, Co, Mn and Al.
In this case, the Co content in the starting powder is preferably 6 wt % or less.
The mean particle size of the starting powder is preferably 5 to 30 xcexcm.
The starting powder generally contains oxygen on the surface thereof and the oxygen content is preferably 1 wt % or less.
It is preferable to conduct an additional step of mixing the starting powder with water prior to the first step, and conduct the first step by using the starting powder in a moistened state.
In this case, it is effective that the additional step is a step of pulverizing coarse particles of a hydrogen storage alloy containing 20 to 70 wt % of Ni under a condition having water to have a mean particle size of 5 to 30 xcexcm.
It is preferred that the first step is a step of immersing the starting powder in the aqueous solution containing 30 to 80 wt % of sodium hydroxide for 0.2 to 3 hours.
It is preferred that the second step is a step of washing with water the powder which has been subjected to the first step until a pH of used water becomes 9 or less.
It is preferred that the third step is a step of adding an oxidizing agent in a water having a pH of 7 or more in which the powder is dispersed. For example, the third step preferably comprises a step of adding, while stirring, a hydrogen peroxide solution in a water having a pH of 7 or more in which the powder is dispersed. It is preferred that the amount of hydrogen peroxide to be added is 0.5 to 15 parts by weight per 100 parts by weight of the powder.
The present invention also relates to an electrode alloy powder produced by the method in accordance with the present invention described above. More particularly, it relates to an electrode alloy powder which contains 3 to 9 wt % of a magnetic substance comprising metallic nickel.
The electrode alloy powder obtained by the production method of the present invention can yield a battery having an excellent high-rate discharge property especially in the low temperature region.
The present invention also relates to a battery including the above-described electrode alloy powder.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.